Not to hijack this but I'm curious about this point. How much light are we actually losing in the viewfinder due to this setup?

Very little. Consider - the whole main mirror is not semi-transparent, only certain areas allow some light to pass through vs. being reflected. Yet, when you look at an evenly illuminated white wall or clear blue sky, you don't see light and dark areas corresponding to the parts of the main mirror that pass light. So, probably less than 2-3% of the total light, and only in select areas, is 'lost'.

Thanks for clarifying this. I was under the impression that the entire mirror was semi-transparent and that this were the explanation why the viewfinders of modern AF cameras are noticeably darker than old 35mm cameras from the pre-EOS times. And they seem to be smaller, even on full frame. 2-3% light loss and and limitation of the semi-transparency to small areas doesn't really explain this.

I think you're right that they're close to the limit on the mirror with current systems. They did increase the lowlight sensitivity of the 1D X, and I think that was accomplished by increasing the actual sensitivity of the pixels on the AF sensor itself.

Not to hijack this but I'm curious about this point. How much light are we actually losing in the viewfinder due to this setup?

Very little. Consider - the whole main mirror is not semi-transparent, only certain areas allow some light to pass through vs. being reflected. Yet, when you look at an evenly illuminated white wall or clear blue sky, you don't see light and dark areas corresponding to the parts of the main mirror that pass light. So, probably less than 2-3% of the total light, and only in select areas, is 'lost'.

Thanks for clarifying this. I was under the impression that the entire mirror was semi-transparent and that this were the explanation why the viewfinders of modern AF cameras are noticeably darker than old 35mm cameras from the pre-EOS times. And they seem to be smaller, even on full frame. 2-3% light loss and and limitation of the semi-transparency to small areas doesn't really explain this.

Viewfinder magnification is a fairly important specification, although it's not discussed too often. The bigger the viewfinder the more you can see, and the more you can see the easier it is to compose and frame a shot. Viewfinder coverage is something Snapsort seems to have left out of their equation. If coverage is less than 100%, you can end up getting things in your final image that you didn't see in the viewfinder.

Yes, the 1D X and 1DsIII have very large viewfinders. That adds I little to the cost, because the pentaprism must be larger to support the larger viewfinder. Viewfinder magnification is one area where the 1D X is clearly better than the Nikon D4.

The 1dm4 contains cross-point sensitive af whit some lenses and 1,4X converter (ex. 70-200 f2.8 II + 1,4X) so it will be cross-point sensitive at f4 -> how do this work? why not sensitive whit all f4 lenses?

That's a good question, and the best answer is probably 'because that's what Canon decided.' The limitations are in the firmware. The trend continues - for example, Canon proudly advertises that the new 1D X has five f/2.8-sensitive dual cross points, but they don't proudly advertise the fact that the 24-70mm f/2.8L, despite being an f/2.8 lens, only activates one of those five dual cross-type points.

I'm not suggesting Canon is being arbitrary or capricious here - most likely, they test these lenses with the AF system and remove firmware support for combinations that don't deliver results they find to be consistent with 1-series performance. The lenses that are f/2.8 but only activate one of five f/2.8 dual crosses have in common that they are all 24mm or wider with an f/2.8 max aperture. Narrower f/2.8 lenses, and faster wide lenses (24mm f/1.4) activate all 5 points. That suggests there is a technical reason for these decisions.

And then the 7d af, is the 7d cross-points only active whit lenses that has an aperture of f2,8 or larger?

No, the 7D has 19 cross-type points with all lenses of f/5.6 or faster. The center point is a 'dual cross' point - an f/2.8-sensitive 'x' superimposed onto the f/5.6-sensitive '+', and the f/2.8 'arms' are active with f/2.8 and faster lenses.

If I remember correctly, the 50D is a dual cross for the centre point as well? But the others are just cross types?So all 9 AF points are cross types sensitive at 5.6 and wider, but the centre point has a X cross thats sensitive at 2.8 and wider as well?

Maybe thats where I got mixed up... Numbers too-apparently

What will happen if an extender is used though? Or something goes wrong wiht the lens and it gets stuck at 7.1 or something... total inability to AF?

...Canon proudly advertises that the new 1D X has five f/2.8-sensitive dual cross points, but they don't proudly advertise the fact that the 24-70mm f/2.8L, despite being an f/2.8 lens, only activates one of those five dual cross-type points.

Any idea if the 70-200 2.8 IS II will activate all five points on the 1D X? Or will all five only work with the 300 or 400 2.8 IS II attached?

if you can find the older kenko DG Teleconverters apparently they will AF because there is no AF confirm like the new DGX ones apparently they are optically the same so these older ones will allow you to AF at greater than f8 even on non 1 series bodies you just have to factor it into your exposure metering thats all.